The formation of chemical synapses is an essential step in development of the nervous system, and one that is likely to be involved in a wide range of neurological and psychiatric disorders. The process of synaptogenesis can be analyzed in particular detail at the vertebrate neuromuscular junction, because of its accessibility to electrophysiological, pharmacological, and biochemical manipulation. Our purpose is to explore how motor neurons regulate the expression of specific genes, and the assembly of their protein products, in muscle cells during development of the neuromuscular junction. In addition, the molecular action of a trophic factor that may mediate these effects will be investigated. This study will make use of a well-established nerve-muscle culture system that permits ready observation and experimental manipulation of nascent synapses. The principal objectives of the project are as follows: 1) To examine by cytochemical methods how the accumulation of synapse-associated proteins such as the acetylcholine receptor, and the mRNAs that encode them, is modulated in regions of the muscle cytoplasm underlying developing synapses. 2) To dissect pharmacologically the relationship between insertion of new AChRs into the subsynaptic membrane, and clustering there of preexisting receptors; both processes are operative during synaptogenesis. 3) To investigate the mechanism of actionof a polypeptide from chick brain that stimulates inserrtion and clustering of AChRs on the surface of muscle cells. Its effect on synthesis and assembly of receptor subunits, and on the amounts of the corresponding mRNAs will be tested. Whether it influences the distribution and synthesis of other synaptic molecules will also be examined. 4) To clone a cDNA encoding the trophic polypeptide using recombinant DNA technology. This clone will be essential for obtaining further structural information about the polypeptide, as well as for determining its sites of synthesis in the developing nervous system.